FLOW BIREFRINGENCE AND COMPUTATIONAL STUDIES OF A SHEAR THINNING POLYMER-SOLUTION IN AXISYMMETRICAL STAGNATION FLOW

Flow of a shear thinning, moderately concentrated solution of monodisp erse high molecular weight polystyrene has been studied in a nonhomoge neous axisymmetric stagnation flow. Shear and extensional rheometry ha ve been used to characterize the rheology of the solution. Multimode G iesekus and Phan-Thien-Tanner (PTT) models, and a single mode PTT mode l with a White-Metzner rate dependent relaxation time (PTT-WM) were fi t to the rheological data. Transient uniaxial elongational measurement s show significant extension hardening, which could only be predicted by the PTT model. The Giesekus model provided the best representation of shear flow data, but substantially underpredicted the measured exte nsional viscosity. Flow birefringence was used to measure integrated a xisymmetric shear and normal stress profiles as a function of position . These measurements were compared with the results of numerical simul ations, performed using an adaptive finite element technique, with the three constitutive models. The computed results with the 3-mode Giese kus model accurately portrayed experimental shear and normal stresses in both forward and rear stagnation flow up to Weissenberg numbers (We ) of O(1). At higher We, the Giesekus model underpredicts normal stres ses in the rear stagnation flow, where elongational gradients dominate . Both PTT and PTT-WM models gave rise to unrealistically large extens ional stresses in the rear stagnation flow even at moderate We which l ed to poor agreement with experimental data. On the basis of these com parisons, it appears that the elongational properties of the solution are intermediate between the Giesekus and PTT, but much closer to the Giesekus, predictions. This is in conflict with the experimentally mea sured uniaxial viscosity, which suggests that the PTT model should be superior in flows with significant uniaxial extension. (C) 1998 Elsevi er Science B.V.